As a factor which lowers the yield of a device in accordance with the ultra miniaturization of a recent semiconductor integrated circuit, there is mentioned the existence of a crystal originated particle (hereinafter, referred to as COP) which is a defect introduced at growth of a crystal which is formed during the crystal growth of a silicon single crystal ingot, a minute defect of oxygen precipitate which is the nucleus of oxidation induced stacking fault (hereinafter, referred to as OISF), or an interstitial-type large dislocation (hereinafter, referred to as L/D).
The COP is a pit originated in a crystal which appears on the surface of a wafer when a silicon wafer which is polished in mirror surface is rinsed by the SC-1 method with a mixed solution of ammonia and hydrogen peroxide. When the wafer is measured with a particle counter, the pit is detected as particles (Light Point Defect; LPD). The COP is a cause for deteriorating electric properties such as, for example, the time dependent dielectric breakdown (TDDB) of an oxide film and the time zero dielectric breakdown (TZDB) of an oxide film. Further, when the COP exists on the surface of a wafer, a bump is generated at the wiring step of a device to be possibly the cause of disconnection. It becomes also the cause of leakage and the like at portion separating an element and the yield of a product is lowered.
It is considered that a minute oxygen precipitation which is formed at crystal growth is the nucleus of OISF, and the OISF is a stacking defect which is exposed at a thermal oxidation step when a semiconductor device is produced. The OISF becomes the cause of trouble of increasing the leak current of the device, and the like. The L/D is also called as dislocation cluster, or also called as dislocation pit because when a silicon wafer generating the defect is immersed in a selective etching solution in which fluoric acid is a main component, etching pit having orientation is generated. The L/D is also a cause of deteriorating electric properties such as, for example, leak property and isolation property.
From the above reasons, it is necessary to reduce COP, OISF and L/D from a silicon wafer which is used for producing a semiconductor integrated circuit.
A defect-free ingot having no COP, OISF and L/D and a silicon wafer which is sliced from the ingot are disclosed in JP-A-11-1393 corresponding to U.S. Pat. No. 6,045,610. The defect-free ingot is an ingot comprising a perfect region [P] when the perfect region in which a agglomerate of vacancy type point defects and interstitial silicon type point defects in the ingot are respectively not detected is referred to as [P]. The perfect region [P] exists between a region [V] having defects where the vacancy type point defect is dominant in an ingot and excessively saturated vacancies are agglomerated and a region [I] having defects where the interstitial silicon type point defect is dominant and excessively saturated interstitial silicons are agglomerated.
Further, it is indicated in JP-A-2001-102385 that the perfect region [P] having no defect where the point defects are agglomerated is classified into a region [Pv] where the vacancy type point defect is dominant and a region [Pi] where the interstitial silicon type point defect is dominant. The [Pv] region is adjacent to the [V] region and a region having a concentration of vacancy type point defect which is less than the minimum concentration of vacancy type point defect which can form a OISF nuclei. The [Pi] region is adjacent to the [I] region and a defect region having a concentration of interstitial silicon type point defect which is less than the minimum concentration of interstitial silicon type point defect which can form an interstitial-type dislocation.
When the pulling-up speed of an ingot is set as V (mm/min.) and a temperature gradient to the vertical direction of the ingot nearby the solid-liquid interface between silicon melt and the silicon ingot is set as G (° C./mm), an ingot comprising the perfect region [P] is prepared within a range of V/G (mm2/min.·° C.) in a region where OISF (P band) which is generated in a ring form at thermal oxidation processing is extinguished at the center of a wafer and L/D (B band) is not generated.
Since it is important to control the concentration distribution of point defect to an axial direction and a diameter direction in order to produce a defect-free silicon wafer, the following method has been conventionally adopted for measuring secondary defect which is generated by thermal processing in an ingot, namely, agglomerated defect distribution. Firstly, a silicon single crystal ingot is prepared at a condition by which a defect-free region is formed, for evaluation of the concentration distribution of point defect to an axial direction and a diameter direction. Then, a sample is prepared by slicing the ingot to an axial direction. Then, after the sample is mirror-etched, it is thermally treated at 800° C. for 4 hours under nitrogen or oxidative atmosphere and thermally treated at 1000° C. for 16 hours, successively. The sample thermally treated is measured by measurement methods such as the copper-decoration, the secco-etching, the X-ray topography and the lifetime measurement. As shown in FIGS. 23 and 24, since oxygen precipitate appears in the ingot by the above-mentioned thermal treatment, respective region and respective boundary have been discriminated and judged from the oxygen precipitates. Hereat, FIG. 23 is a chart showing the in-plane distribution of recombination lifetime when the thermal heat treatment for oxygen precipitation is carried out for a sample containing the regions [V], [Pv], [Pi] and [I] which is prepared by changing the pulling-up speed from a high speed to a low speed by a crystal pulling-up apparatus having a hot zone which can prepare a defect-free silicon single crystal ingot, and FIG. 24 is a chart showing the recombination lifetime in the A-A line section of FIG. 23.
However, in case of the above-mentioned measurement method, it is known that the measurement value of recombination lifetime is remarkably dependent on the oxygen concentration in a sample and the thermal heat treatment for oxygen precipitation condition. For example, when the concentration of oxygen dissolved in an ingot is low, the density of the oxygen precipitate which is formed by the thermal heat treatment for oxygen precipitation is low in comparison with a sample in which the concentration of dissolved oxygen is high; therefore the difference value of the recombination lifetimes becomes small.
On the other hand, when the concentration of oxygen dissolved in an ingot is high, the amount of oxygen precipitates is also excessively large depending on the concentration of oxygen dissolved in the regions [Pv] and [Pi] and the difference between the measurement values of diffusion lengths of minor carrier in the regions [Pv] and [Pi] is small; therefore the difference is not clear. Thus, there has been a problem that the boundary region between the region [Pv] and the region [Pi] cannot be cleared in low oxygen concentration and high oxygen concentration.
Further, thermal treatment for a long time is not only required for the thermal heat treatment for oxygen precipitation, but also the condition of the thermal treatment affects the oxygen precipitation in an ingot; therefore the boundary region of the point defect regions is shifted to either region side of the region [Pv] or the region [Pi]. Consequently, it is difficult to discriminate and judge the original point defect region.
It is the purpose of the present invention to provide a method for measuring the point defect distribution of a silicon single crystal ingot which identifies the regions [Pv] and [Pi] in an ingot and boundary thereof with high precision and a short time without depending on the concentration of oxygen dissolved in the ingot.